As explained in the patent of Wilcox and Milkovitz, encoder strips or "codestrips" are used in image-related devices such as printer/plotters, scanners, facsimile machines and the like. A codestrip helps establish the position of a marking or sensing device that is mounted for scanning across a printing medium on which an image is to be printed, or from which an image is to be read.
The word "scan" (with its related term "scanner") is commonly used in two senses. A first meaning refers to travel of a carriage transversely across an image that is being either printed or sensed. The carriage holds a marking or sensing device that either forms or reads the image, respectively. In this type of apparatus the transversely scanning device forms or reads one line or swath of pixels across the image, and then the image-bearing page or sheet is moved in an orthogonal direction--lengthwise--so that the marking or sensing device can form or read the next line or swath. In this way eventually the entire image is formed or read.
The second meaning refers much more specifically to reading, not forming, an image. In this sense, "scan" (or "scanner") applies to devices that read images, whether or not they have any scanning motion in the first sense described in the preceding paragraph, above. Thus a "scanner" may employ a long linear array of sensing elements that extends all the way across an image--so that there is no need for transverse motion. Usually in this type of scanner an image-bearing page moves lengthwise past the sensor array, and the scanning is in this direction only.
The present invention relates only to devices that scan in the first sense described above. This invention relates to determination and control of carriage position and velocity, for the carriage which holds the marking or sensing device.
A codestrip is a graduated strip, generally disposed across an area where the medium is held, and having graduations that can be automatically sensed. Historically codestrips have been made of polymeric material such as that known commercially as Mylar.RTM., with graduations formed photographically.
For optimum performance, the codestrip graduations should be very close to both a light source and a detector used as parts of a sensing system to read the graduations. This condition has been met most effectively by threading the codestrip through a narrow transverse aperture in the transversely scanning carriage.
Immediately to one side of the aperture is an optical source and immediately to the other, a detector. The codestrip is tensioned, to at least minimize its rubbing or flapping against the rapidly moving surfaces of the narrow aperture as well as to provide reasonable straightness (and thus a systematic uniformity in the readings).
These polymeric strips serve well in desktop devices for dealing with printing-medium sheets of about twenty centimeters (81/2 inch) in width--or somewhat larger, up to for example about thirty centimeters (twelve inches). For progressively wider images, however, due to various factors detailed in the above-mentioned patent, polymeric strips have been progressively less satisfactory.
Such factors include expansion and contraction due to temperature and humidity, and stretch or "creep" with elevated temperature and increasing tension. In a large-format machine for dealing with images some fifty-five centimeters (twenty-two inches) across, these deficiencies are very difficult to manage in an economic way.
Wilcox and Milkovitz responded to these difficulties with a codestrip made entirely of metal, about 0.051 millimeter (0.0020 inch) thick. The scale graduations are a series of extremely fine orifices, etched through the metal.
This type of strip performs superbly in terms of machine operation--but etching through metal inherently produces orifice boundaries which are not as accurate as the boundaries of graduations (lines) that can be formed on film. As will be understood, this limitation arises in part because the thickness of the strip, for a reasonably strong strip, is not much smaller than the width of the windows.
For example Wilcox and Milkovitz describe a strip with 0.051 mm thickness and 0.08 mm windows. Thus etching tends to undercut the resist, or otherwise modify the structure of the strip--irregularly, leading to nonreproducible edges--as well as forming intended orifices.
This characteristic limits the image resolution that can be achieved using a metal codestrip, with little room for advancement in this aspect of the art. In addition the metal strip may present a risk of injury through unintended contact with thin metal edges. Also the metal strip is somewhat less versatile or readily modified.
With regard to versatility, because these codestrips are so long a very large tool or fixture is needed to make several strips. As a result, even a small change entails large expenses in retooling.
For example in some printers or plotters for operator convenience it is desirable to lengthen the travel of the pen carriage to accommodate related devices--for example a pen-refilling station--inside the case of the apparatus. More generally, in a commercial context it is desirable to offer printer/plotter products having various levels of performance and cost.
These simple sorts of changes do not actually implicate the encoding function of the codestrip, but merely require it to be slightly longer or shorter. With a metal codestrip, however, even such simple mechanical modifications become a significant project in terms of retooling cost.
Furthermore, continuing use and maintenance of a separate custom tool is likely to be needed, for each such minor variation among strips. Some of these diverging requirements can be avoided by thoughtfulness in design, but usually at additional cost or inconvenience of some other sort--to circumvent the different mechanical constraints.
In summary a metal strip is much less versatile or readily modified to suit varying requirements than a film strip, which can be fabricated from any new master pattern quickly and easily, even in small quantities, by inexpensive photographic techniques.
In use of metal codestrips a more fundamental limitation arises from the metal-etching technology that is used to form the apertures or windows. The higher the resolution desired, of course the smaller the window width needed.
To maintain a reasonably orderly control over shapes during etching, however, as mentioned above the thickness of the metal strip must be on the same order as the window width, and preferably at least somewhat smaller. Thus higher resolution in the pixel grid of the plotter demands a thinner metal band.
Using a significantly thinner strip introduces greater metal creep, impairing a major benefit of the metal-strip system over plastic--namely dimensional stability. In addition a thinner strip may heighten any risk of injury in the field, and requires much greater care to avoid breakage during assembly and testing.
The potential for breakage leads to a requirement for great care and relatively difficult manipulations in assembly, testing and service--all translating into hidden costs or other impediments for higher-resolution plotters. Thus the metal codestrip tends to impede the progress of the art toward higher performance and particularly higher resolution.
Another known form of codestrip involves use of a scale that is formed on film, such as for example a polymeric film. The film scale is glued to a rigid panel or plate of the apparatus case or chassis.
While such a system has some advantages, it is undesirable in that the scale is not readily and economically brought into very close proximity with the sensing system that must read the scale.
Thus important aspects of the printing technology used in the field of the invention remain amenable to useful refinement.